Method and device for doping, diffusion and oxidation of silicon wafers under reduced pressure

ABSTRACT

Method and device for doping or diffusion, or oxidation of silicon wafers ( 4 ), the wafers being introduced into the chamber ( 2 ) of an oven ( 1 ) wherein is introduced at least a gas for performing the doping or diffusion or oxidation process. The method comprises simultaneously with the introduction and passage of gas into the chamber ( 2 ) of the oven ( 1 ), continuously subjecting the latter to a depression of constant value. The device comprises an oven ( 1 ) provided with a chamber ( 2 ) wherein are introduced the wafers, the oven including at least an inlet tube ( 5   a,    5   b,    5   c ) for introducing at least a gas into the chamber ( 2 ) to carry out the processes and at least an outlet tube ( 6 ) for extracting the gas whereto is connected a suction unit ( 7 ) for generating in the chamber ( 2 ) a constant and controlled depression.

The present invention relates to N and P type doping, diffusion andoxidation of silicon wafers in the process for production ofsemiconductors.

In the process for the production of integrated circuits forelectronics, the techniques of fabricating semiconductors are basedprincipally on the intrinsic modification of the atoms of the siliconmaterial. The insertion in the silicon lattice of elements with asurplus of electrons, of the so-called N type, and a deficit ofelectrons, of the so-called P type, is defined by the term doping of theN type or the P type.

To render silicon semiconductive, it is necessary to add to it so-calleddoping elements.

The known doping elements are for example phosphorus, arsenic, antimony,boron, gallium or aluminum. In the processes most often used for dopingwith phosphorus or boron for example, the silicon wafers are introducedinto an oven and brought to a temperature comprised generally between800° C. and 1200° C. This temperature is necessary to permit asuperficial concentration of the phosphorus or the boron on the surfaceof the silicon wafer.

The sources of solid, liquid or gaseous dopants are vaporized in thechamber of the oven from a carrier gas in contact with the source ofdopants. The carrier gas plays the role of accelerating the doping ofthe dopant gas on the surface of the silicon wafer. This type of deposittakes place in the oven at atmospheric pressure. To complete thedistribution between the wafers, the carrier gas must be supplied at ahigh gas vector flow rate, which gives rise to a large consumption ofgas.

The horizontal or vertical ovens used generally have a chamber oftubular shape in which the silicon wafers are disposed on a quartz orsilicon carbide support. One of the ends of the tubular chamber isprovided with a door to permit the introduction of the wafers. The otherend is closed by a non-removal end wall. The doping gas, the reactinggas and the vector gas are generally injected through an openingprovided in the end wall of the tubular chamber.

To obtain good uniformity of treatment, it is necessary to limit thenumber of wafers in the oven, generally not more than 50, and to leave arelatively large space between each wafer.

The use of a vector gas also gives rise to problems of substantialcondensation of acid in the oven and the environment. This aciddeposition from the dopant will have repercussions on the repetitivityof the treatment of the wafer but also on the different externalconstituent elements of the oven, which requires frequent disassemblyfor cleaning of the oven and particularly of the quartz tube comprisingthe chamber of this latter.

On the other hand, quartz diffusers, to improve the result, arenecessary, increasing the costs of use and control of the quartzportions. The returned operation after cleaning requires preliminarytests giving rise to loss of productive capacity. Other phenomena suchas the lack of uniformity on the wafer and on the load and from load toload, a memory effect of doping, parasitic inclusions of dopants in thecrystallographic lattice of silicon because of excess oxygen,modifications of lifetime of the minority carriers, the limitation ofthe diameter of the wafers, are also factors requiring a more burdensomechoice of equipment.

The present invention has for its object to solve the problems mentionedabove by providing a new process of doping, diffusion and oxidation.

The process according to the invention is essentially characterized inthat it consists in conjointly with the introduction of the gas into theoven chamber, subjecting this latter to reduced pressure. This effectpermits increasing the speed of the gas in the chamber and renders nolonger necessary the use of large quantities of vector gases. Only thereactive gases are present in the silicon doping chamber.

According to another characteristic of the invention, the value of theunder pressure is comprised between 100 and 800 millibars. This range ofvalues is compatible with relatively simple means for the creation ofthe under pressure.

The invention also relates to a technique for injection of oxygen whichpermits dosing with precision the quantity of oxygen to limit the speedof cracking of the dopant, which is to say is molecular decomposition.

The invention also relates to the device permitting carrying out anoperation of doping, diffusion or oxidation of the wafers. The devicecomprises an oven provided with a chamber hermetically closed by a door,into which chamber are introduced the wafers, said oven comprising atleast one injection tube for at least one gas into the chamber to carryout one of the mentioned operations.

The device as defined is characterized essentially in that it comprisesmoreover at least one gas extraction tube, an aspiration means connectedto the gas extraction tube, adapted to create in the chamber a constantand controlled under pressure.

Thus, the use of large quantities of vector gas of the nitrogen type isno longer necessary because of the under pressure that is created.Moreover, the peripheral condensation of acid is greatly reduced, theacid being in large part drawn into the aspiration means. The residualcondensation will be itself recovered by added devices such as tiles orthe like.

The present invention also has for its object a device for doping,diffusion or oxidation of silicon wafers, comprising an ovenhermetically closed by a door, into which chamber are introduced saidwafers, said oven comprising at least one introduction tube for at leastone gas into the chamber to carry out the mentioned operations.

Other advantages and characteristics of the invention will becomeapparent from the reading of the description of a preferred embodiment,given by way of non-limiting example and illustrated by the accompanyingdrawings, in which:

FIG. 1 shows schematically the device for doping, diffusion or oxidationof silicon wafers according to the present invention,

FIG. 2 shows in longitudinal cross-section a device for doping,diffusion or oxidation, according to the invention.

As shown, the device for doping, diffusion or oxidation of siliconwafers according to the invention comprises an oven 1 provided with asealed chamber 2 associated with heating means 3, and into which areintroduced silicon wafers that are to be subjected to the mentionedoperations. The oven 1 comprises at least one introduction tube 5 a, 5b, 5 c, for at least one gas into the chamber 2, to achieve thementioned operations and comprises moreover at least one gas removaltube 6, and suction means 7 connected to said gas removal tube 6,adapted to create in the chamber a constant and controlled underpressure, this suction unit being located at a distance from the oven 1in a temperate region.

The chamber 2, for example of tubular shape, is hermetically closed by afixed end wall 2 a and by a door 2 b installed at its ends, the door, inthe closed position, and the end wall being perpendicular to thelongitudinal axis of symmetry of the tubular chamber.

The heating means 3, constituted for example by electrical resistances,are distributed or wound about the tubular chamber and are disposedspaced from this latter or else in contact therewith.

Preferably, the tubular chamber 2 has a cross-section in the form of acircular crown. This arrangement gives the wall of the chamber goodmechanical resistance to atmospheric pressure when the internal volumeof the chamber is subject to under pressure.

The door 2 b is secured to a mounting slidably engaged on a suitablesupport. The opening and closing movements of the door take place alongthe longitudinal axis of the tubular chamber. With the door isassociated a cover extending outwardly of the chamber in which ismounted the shaft of a blade for loading wafers. This blade receiveswafer supports. Preferably with this cover is associated an means foradjusting the inclination of the blade and said cover is mounted withthe possibility of pivotal movement about a horizontal axis normal tothe longitudinal axis of the blade. By action on the adjustment means,the blade, under vacuum, is more or less upwardly inclined, such thatwhen the latter is loaded it can substantially come into positionsubstantially along a horizontal plane. This adjustment can be carriedout with a weight on the blade, the weight of this load corresponding tothe weight of the load to be treated.

Preferably, the door 2 b of the oven chamber is constituted of opaquequartz or is opacified and has a suitable sealing joint made of amaterial based on a fluorinated elastomer. Such a material isparticularly known under the commercial name “VITON”. Such a material ismade by the DUPONT DE NEMOURS company.

The opaque quartz constitutes an effective barrier to infrared, burningof the sealing joint is thus avoided. So as further to reinforce theprotection of the sealing joint, the terminal portion of the tubularchamber 2 (the one adapted to receive the door 2 b) is made of opaquequartz. In practice, this opaque portion is assembled with thetransparent door of the chamber by any means known to those skilled inthe art. In the preferred embodiment, this assembly is made by welding.Again so as to reinforce the thermal protection of the joint, a thermalscreen is disposed in the chamber facing the door.

The door, by its sealing joint, is applied against an annular edge ofthe opaque terminal portion of the tubular chamber, this annular edgebeing also opaque. The annular surface of this edge, facing the joint ofthe door, constitutes a joint plane and receives the joint underpressure. This joint is preferably mounted in an annular throat hollowedout of the door 2 b. To absorb defects of parallelism between the doorand the joint plane, said door is mounted floatingly on the mounting,which is to say with limited latitude of movement about two secant andorthogonal axes parallel to the plane of the joint and along an axisnormal to this joint plane. Preferably, between the door and itsmounting are disposed one or several resilient members and the covercomprises an axially deformable portion.

To resist heat, the walls of the oven, namely, the tubular wall of thechamber 2 and the terminal wall 2 a, are constituted of quartz. Thetubular chamber could be provided with an internal covering of siliconcarbide of tubular shape. This covering resists radial deformationinwardly of the tubular chamber 2, particularly when this latter issubjected to high temperature.

The path of the gas in the chamber 2 of the oven 1 is established froman upstream region 8 toward a downstream region 9 located at a distancefrom the former, the silicon wafers 4 being disposed between these tworegions along the path of the gases. These wafers 4 are disposed in thechamber 2 of the oven transversely to the direction of flow of the gasesin this latter. Thus there is created a relatively great pressure dropthanks to which the gases can sweep the surfaces to be treated of thewafers.

The wafers 4 are installed on a removable support 10 introduced into thechamber 2 of the oven. This support is made of a material adapted toresist both the heat as well as the corrosive power of the gases. Thusthis support could be constituted of silicon carbide or else of quartz.The support and the wafers, after positioning in the chamber, aredisposed at a spacing from the end wall 2 a and from the door 2 b, whichprovide the upstream region 8, this latter being located between the endwall 2 a and the position of the wafer support, and the downstreamregion 9 located between the door 2 b and said position, the gas flow insaid chamber 2 being established between one and the other zone and theregion of the chamber corresponding to the position of the silicon wafersupport being the hottest zone of said chamber 2.

The introduction tube or tubes 5 a, 5 b, 5 c of the gas or gases in thechamber 2 of the oven pass through the end wall 2 a of this latter orthe door 2 b and flow into one or the other upstream region 8 ordownstream region 9 of the withdrawal tube 6, gases passing through theend wall of the oven 2 a or the door 2 b and flowing into the otherupstream or downstream zone 8 or 9 of the chamber 2. According to thepreferred embodiment, the tube or tubes 5 a, 5 b, 5 c for introductionof gas into the chamber 2 and the withdrawal tube 6 of the gas, passthrough the end wall 2 a of the chamber, and the introduction tubes 5 a,5 b, 5 c of the gas open into the upstream region 8 of the chamberwhilst the withdrawal tube 9 for the gases opens into the downstreamregion 9 of said chamber 2.

One of the tubes 5 c for introduction of gas into the chamber 2 receivesa reactive gas of the type of oxygen. Preferably, this tube 5 c opensinto the warmest region of this latter, nearest the position occupied bythe wafer support, to avoid the anticipated cracking of the doping gas.

As mentioned above, the device according to the invention is providedwith a unit 7 for sucking out gases from the chamber 2, adapted tocreate continuously an under pressure of constant and perfectlycontrolled value. This under pressure has the object of increasing thespeed of the gases in the chamber and avoiding the use of vector gases.

According to the preferred embodiment, the suction means 7 comprisesparticularly a suction pump 11 preferably of the membrane type. Thispump 11 at least for its members in contact with the gas, will be madeof a material adapted to resist the corrosion of said gases. There couldbe used a material constituted of tetrafluoroethylene. Such a materialis known under the commercial name of “TEFLON”.

Again according to the preferred embodiment, the suction unit 7comprises control and regulation members of the under pressure in theoven chamber, these control and regulation members being incommunication through conduits on the one hand with the suction tube ofthe pump and on the other hand with the removal tube from the oven.

The control and regulation members comprise particularly a over pressureor ballast, constituted by a cylindrical chamber connected by conduits,on the one hand to the suction tubing of the pump and on the other handto the tubing 6 for withdrawal from the oven, said overfilling capacity12 being moreover connected to a source under pressure of over pressuregas, controlled by means of a control valve 13, directed to open andclose by a circuit for measuring and controlling the dynamic underpressure in the chamber 2 of the oven 1.

This measuring and control circuit of the under pressure comprises apressure detector 14 disposed outside the chamber, in the connectionconduit extending between the over pressure capacity and the withdrawaltube of the oven or else to the over pressure capacity. This pressuredetector 14 is adapted to produce a signal of an intensity proportionalto the value of the under pressure, this signal being applied to theinlet of the compressor 15 which compares the value of this signal witha reference value. This comparator is connected by a power circuit tothe control valve 13 for, as a function of the spacing between the valueof the under pressure and the reference value, adjusting the degree ofopening of the valve member of the control valve 13 and hence the flowrate of the over pressure gas or controlling the closing of this closuremember of the control valve, which permits by over pressure more or lesspronounced, from the pump or by the absence of over pressure, adjustingat each instance the value of the under pressure in the chamber 2 of theoven. Stated otherwise, the suction capacity of the pump being constant,the increase of flow rate of the over pressure gas is accompanied by adecrease of the flow rate of the gases extracted from the chamber of theoven, and vice versa.

The over pressure capacity 12 receives over pressure gas at ambienttemperature and the hot gas is withdrawn from the oven chamber. Themixture of the gases in this over pressure capacity gives rise tocondensation which is recovered by this capacity. Thus this condensationcan in no way disturb the operation of the pump 11. The over pressurecapacity will be removable, particularly for extracting the variouscondensations.

It will be seen that the degree of under pressure in the oven chamberand the control of this latter, take place outside of the oven inrelatively cool and temperate regions.

The gases rejected by the pump are then neutralized with the help of anyappropriate means.

The advantages of the invention are as follows:

-   -   compatibility with atmospheric technology,    -   uniformity of the treatment of each wafer, of each load        (assembly of plates on a support), and from load to load,    -   repetitivity,    -   reproductivity after stopping,    -   suppression of the memory effect,    -   suppression of overdoses of dopants,    -   no limitation on the diameter of the wafers,    -   considerable reduction of maintenance,    -   reduction of costs,    -   peripheral cleanliness,    -   frequency of cleaning greatly decreased.

It follows that the teachings of the present invention are equallyapplicable to an oven of the horizontal type as to an oven of thevertical type. It is quite evident that the present invention can be thesubject of any arrangement and modification within the field oftechnical equivalents, without thereby departing from the scope of thepresent application.

1. A process for doping, diffusion, and oxidation of silicon wafers,said process comprising: placing said wafers on a removable support andintroducing said support into a chamber of an oven that is hermeticallysealed by a door and then heating said oven, and introducing gases intothe chamber to accomplish the doping, diffusion, and oxidation, whereinthe gases are a doping gas and a reacting gas, to the exclusion of anyother gas, and said gases are introduced into the chamber under-pressureof a constant value.
 2. The process of claim 1, wherein the gases flowfrom an upstream region located between an end wall of the oven chamberand the position of the wafer support to a downstream region locatedbetween the door and said position of the wafer support, the wafersbeing placed perpendicularly between said upstream and downstreamregion, corresponding to the hottest region in said oven chamber.
 3. Theprocess of claim 2, wherein the gases are introduced into the upstreamregion of the oven chamber through the end wall of the oven chamber, andthe gases are extracted from the downstream region of the oven chamberthrough the end wall of said chamber.
 4. The process of claim 3, whereinthe reacting gas is introduced into the oven chamber through a tubeyielding into the warmest zone of said oven chamber, nearest theposition occupied by the wafer support, to avoid cracking of the dopinggas.
 5. The process of claim 1, wherein the value of the pressure in theoven chamber is comprised between 100 and 800 millibars.
 6. The processof claim 1, wherein the gases are extracted from the oven chamber bysuction with a suction pump of the membrane type.
 7. The process ofclaim 6, wherein the pressure in the chamber is controlled and regulatedby a measurement and control circuit of the under-pressure in the ovenchamber by an over-pressure or ballast capacity, connected by conduitsto the suction tube of the pump or to the withdrawal tube of the gasesfrom the oven chamber, said overpressure capacity being connected to asource of pressurized gas controlled by a control valve for the openingand closing by a measurement and control circuit of the under-pressurein the oven chamber.
 8. The process of claim 7, wherein the measurementand control of the pressure are effected by a pressure detector disposedoutside the oven chamber, in the connection conduit extending betweenthe oven pressure capacity and the withdrawal tube from the oven, saidpressure detector being adapted to produce a signal of an intensityproportional to the value of the actual pressure, said signal beingapplied to the inlet of a comparator which compares the value of thissignal to a reference signal, said comparator being connected by a powercircuit to the control valve for, as a function of the gap between theactual pressure value and the reference value, adjusting the degree ofopening of the control valve or controlling the closing of the controlvalve which permits by more or less pronounced feeding up of the pump orby the absence of feeding up to adjust at each instant the value of thepressure in the chamber of the oven.
 9. A device for doping, diffusion,and oxidation, of silicon wafers, said device comprising: an ovenprovided with a chamber hermetically sealed by a door, said ovencomprising an emplacement for introduction of a removable support onwhich said wafers are placed that defines an upstream region locatedbetween an end wall of said oven chamber and said wafer support, and adownstream region located between the door of the oven chamber and saidwafer support, said end wall of the oven chamber comprising inlet tubesfor introduction of a doping gas and a reacting gas into the ovenchamber, in the absence of means for introduction of any other gases,and one tube for the withdrawal of the gases, connected to a suctionunit, that is adapted for controlling and regulating the pressure in theoven chamber, and to create in the oven chamber a constant andcontrolled under-pressure, and yielding to a stable and controlled flowof the gases in said oven chamber.
 10. The device of claim 9, whereinthe inlet tubes for introduction of gases into the oven chamber passthrough the end wall of the oven chamber and open into the upstreamregion and the withdrawal tube for the gases passes through the end wallof the oven chamber and opens into the downstream region of the ovenchamber, so that the gases flow from the upstream region to thedownstream region of the oven chamber.
 11. The device of claim 9,wherein the inlet tube for introducing the gases into the oven chamberopens nearest the position occupied by the wafers, that is the warmestzone of the oven chamber.
 12. The device of claim 9, wherein the suctionunit comprises a pump of the membrane type.
 13. The device of claim 12,wherein the pump, at least as to parts of the pump in contact with thegases, is made of a material adapted to resist corrosion by said gases.14. The device of claim 12, wherein the suction unit comprises controland regulation members for the pressure in the oven chamber, saidcontrol and regulation members being in communication by conduits with asuction tube of the pump or the withdrawal tube of the oven chamber. 15.The device of claim 14 wherein the control and regulation memberscomprise an overpressure or ballast capacity, connected by conduits, tothe suction tube of the pump or to the withdrawal tube for withdrawingthe gases from the oven chamber, said over-pressure capacity beingconnected to a source of pressurized gas controlled by a control valvefor the opening and closing by a measurement and control circuit of theunder-pressure in the oven chamber.
 16. The device of claim 15, whereinthe measuring and control circuit for the under-pressure comprises apressure detector disposed outside the oven chamber, on a connectionconduit extending between the overpressure capacity and the withdrawaltube, said pressure detector being connected to a comparator, saidcomparator being connected by a power circuit to the control valve. 17.The device of claim 9, wherein the oven chamber has a tubular shape,with a cross-section in the shape of a circular crown.
 18. The device ofclaim 9, wherein the walls of the oven chamber are made quartz and saidoven chamber has a tubular chamber that receives an internal cladding ofsilicon carbide.
 19. The device of claim 9, wherein the door and the endportion of the oven chamber are made of opaque quartz.
 20. The device ofclaim 9, wherein the door is secured to a mounting engaged slidably on asupport and said door is mounted floatingly on the mounting, so thatsaid door has a limited latitude of movement about two secant andorthogonal axes parallel to a joint plane of the door with the ovenchamber and along an axis normal to the joint plane.